1. Field of the Invention
This invention relates to a MOS type semiconductor memory device, and, in particular, to a reprogrammable semiconductor memory device, such as an EPROM or EEPROM.
2. Description of the Prior Art
A reprogrammable semiconductor memory device, such as an electrically programmable read only memory or simply EPROM, is well known in the art. A typical prior art EPROM is shown in FIGS. 3a and 3b in plan and cross sectional views, respectively. An EPROM typically has a two-layer polysilicon structure and the illustrated EPROM includes a silicon substrate 2, a gate oxide 4 formed on the silicon substrate 2, a floating gate 6 formed by a first layer of polysilicon, an interlayer oxide film 8, and a control gate 10 formed by a second layer of polysilicon. Also provided in the structure shown in FIG. 3b is a field oxide 12 formed for example by the LOCOS (local oxidation of silicon) method. Also defined in the silicon substrate 2 as shown in FIG. 3a - is a source region S and a drain region D.
In such an EPROM, a capacitance C.sub.2 between the control gate 10 and the floating gate 6 is generally required to be approximately three times larger than a capacitance C.sub.1 between the floating gate 6 and the channel region of the substrate 2. For this reason, the floating gate 6 is typically formed to significantly extend over the field oxide 12. As an example, assuming that the structure shown in FIG. 3a has a channel region whose width W is 2 microns and length L is 3 microns, the area required for the floating gate 6 to extend over the field oxide 12 will be 2 microns.times.3 microns at each side of the channel region As a result, the area occupied by a single memory region indicated by the one-dotted line M will be 2 microns.times.12 microns, which is rather large. The above arguments also hold true for an electrically erasable PROM or EEPROM. As set forth above, since the floating gate 6 extends horizontally over the field oxide significantly, there has been a difficulty in making a reprogrammable memory device compact in size or high in density.